Standard door openings count as 20 sq ft and windows as 15 sq ft. These deductions are conservative, the drywall still needs to be cut around openings, and the cutoffs are rarely reusable. The 10% waste default covers normal cuts and corner waste for a simple rectangular room. For rooms with large archways or detailed millwork, raise it to 15%. You can also use the Square Footage Calculator to confirm your room dimensions before entering them here.
Tom is drywalling a 12×10 ft bedroom with 9 ft ceilings, 1 door, and 2 windows.
Sheet count alone does not tell the full material story. Choosing the wrong thickness can mean failed inspections and costly tear-outs. The International Residential Code (IRC) specifies minimum panel thicknesses based on framing spacing and the application type.
| Thickness | Primary Use | Key Note |
|---|---|---|
| 1/4" | Overlay over existing drywall or curved walls | Not for structural use |
| 3/8" | Curved walls and remodeling patches | Flexible; limited application |
| 1/2" | Standard walls in residential construction | Most common choice |
| 5/8" | Ceilings, fire-rated assemblies (Type X), garages | Required by many local codes |
Type X (5/8") drywall contains glass fibers that delay fire penetration for one hour in a single-layer application. Many jurisdictions require it between attached garages and living spaces. If you are also calculating roof framing dimensions, the Roof Pitch Calculator handles slope, rafter length, and angle.